1. Field of the Invention
The present invention relates to a production management system for controlling the production of various industrial products such as semiconductor components, e.g., LSI, clothes, and automobiles. The term "production management" or "management of manufacture" as used herein includes (1) so-called scheduling for the determination of order of manufacture of multi-kind products depending on respective process steps or degrees of urgency, (2) monitoring progressive condition of manufacture, and (3) collection of processing data upon manufacture.
2. Description of the Prior Art
Conventional production management system will be described taking an example of manufacturing process steps for manufacturing an LSI.
FIG. 1A is a flow chart schematically illustrating a manufacturing procedure for manufacturing an LSI, and FIG. 1B is a cross sectional view showing a semiconductor substrate being processed for manufacturing a bipolar LSI, with illustrating in more detail the step of photolithography illustrated in FIG. 1A.
Generally, various technologies are applied to process a monocrystalline wafer at least once, and up to several tens times on the larger side in order to make up a fine structure on the semiconductor substrate so that it can exhibit desired functions, thus completing manufacture of an LSI. The technologies include formation of an insulation film such as an oxide film/nitride film, photolithography, diffusion by heat treatment, pattern formation by photolithography, film formation by vapor deposition or sputtering, etching, introduction of impurities by ion implantation, scribing (division), mounting, bonding, etc. As illustrated in FIG. 1A, LSI manufacturing procedure includes a plurality of steps of processing a monocrystalline wafer 1, for example, for obtaining a bipolar LSI, steps of oxidation 2, photolithography 3, burying diffusion 4, epitaxial growth 5, oxidation 6, . . . , photolithography 7, base diffusion 8, photolithography 9, emitter diffusion 10, photolithography 11, metal deposition 12, photolithography 13, alloying 14, scribing 15, mounting 16, bonding/inclusion 17, and product test/reliability test 18 to obtain a product 19, and for obtaining a MOS LSI, steps of photolithography 20, source drain diffusion 21, epitaxial growth 22, oxidation 23, . . . , photolithography 24, metal deposition 25, and photolithography 26, instead of the steps 3 to 13 in the manufacture of a bipolar LSI, to obtain the product 19.
Each technology in the procedure illustrated in FIG. 1A is subdivided into several treatments. For example, the photolithography is comprised, as illustrated in FIG. 1B, by (a) coating of a resist which is a photosensitive composition: a silicon substrate 30 having an oxide layer 31 is provided with a photoresist film 32 on the surface of the oxide layer; (b) alignment of a mask/exposure: a mask 33 having an imagewise pattern is arranged in alignment above the photoresist film 32, light is irradiated through the mask 33 so that the photoresist film 32 is imagewise exposed, and the exposed portion is dissolved in a developer to transfer the pattern; (c) pattern formation by development: patterning is performed by photolithography (d) processing by etching: the exposed portion of the oxide film is etched off, etc. Unit of such procedure is usually called "step". LSIs can be fabricated by repeating such steps several hundreds of times. List of the procedure of the manufacturing process steps is a "process table" as shown in FIG. 2. The process table describes contents of process which include at least machines used, recipes, etc. in the order of process.
The process table is assigned to each lot, i.e., process unit: usually for each cassette, or for each sheet in case of a sheet-fed process), and LSIs can be fabricated by performing the process in order as prescribed in the process table. What is characteristic here is that in the process, the same machine is used more than once, usually, many times, under different conditions or recipes. In case different types of machines are used or different types of products are to be fabricated, fabrication in the same step proceeds under different recipes. The term "recipe" as used herein refers to conditions including not only manufacturing conditions such as oxidation or annealing temperature, gas flow rate, and kind of mask used upon exposure, but also testing conditions such as position at which measurement of size is performed. Thus, the manufacturing process for LSI is a very complicated manufacturing line whose control is far more complicated since it involves many elements and a permutation/combination of many elements must be controlled.
Recently, diversification of products, and immediate response to needs of customers have been desired not only in the field of LSI but also in various other fields. Accordingly, demands in LSI production management are shifting from management for increasing the production efficiency by automation of transportation to delivery date management, and TAT (turn around time) management. In accordance with increased diversification of products, there has been an increasing tendency that many types of products are manufactured in a single production line. In addition, not only products differ in their throughputs but also processes differ in their importance and priority from kind to kind. Importance depends on added value such as design costs for masks, etc., manufacture costs for special steps, and is determined usually when normal lots are input. On the other hand, priority, which indicates which one should precede when a plurality of steps are competitively awaiting for a process to be performed in the same machine, is determined depending on allowance till delivery date. Thus, even when a lot has been assigned low priority since a full allowance in time till delivery date was expected originally, it will be often the case that the priority of the lot must be increased because of a delay from the previous arrangement or schedule which occurred in the midway, or conversely, the priority of the lot must be decreased due to the occurrence of ample time margins for delivery.
It is useful to collect and monitor process execution data not only for preventing the occurrence of faulty products but also for the analysis of causes of troubles when such occur. Therefore, it is indispensable to collect process data including test data. It is desirable but not mandatory to collect the process data automatically from the machines. Upon collection of process data and test data, the same machine is used many times repeatedly in the step concerned, and hence key (keyword) information is attached to the data that enables identification of a particular step in a particular lot, i.e., indication of the position of the step concerned in the process table, before the data can be stored. It is a simpler and easier way to use a process schedule being executed as the key information. More specifically, upon lot processing, a particular lot is recognized by a bar code reader, the process schedule of the lot is received through a network communication, and stored together with the process data in a data base. Alternatively, a magnetic card is transported together with a lot and the card is read to obtain a key information, which is then stored together with process data in a database. These methods are featured in that they can serve as lot tracking management as well. As a procedure for such a conventional production line management, there can be cited, one as illustrated in FIG. 3. In the conventional example illustrated in FIG. 3, the results of schedule in the form of a schedule table (program) is distributed to a computer, and an operator executes the process in accordance with the work instruction based on the program. Processed records such as the process data are fed back to the scheduler by collection of on-line data in which such data are transmitted directly from the manufacture machines and test machines, respectively, or by off-line input of progress data from a terminal by the operator to control the progress and reflect the progress condition in next scheduling.
Mass production line, in which the same kind of products are manufactured in large amounts, mostly uses, as a manufacturing method, a first-in/first-out (FI/FO) heaping method by which older lots selected from jamming lots that stay in each machine in the line and await for a process are processed in order chronologically with the oldest one being processed at first, or a "kanban" system in which excessiveness or insufficiency of received lots is displayed visually. Results of the processing are given indicating which step in which lot the results are concerned with, in situ with keyboard or bar code reading information, before they can be stored in a data base.
In this case, the work-in-process volume of the lots in the line is controlled. For example, SEMI Technology Symposium 92, p. 129 describes a method in which such a control of work-in-process volume is applied to average process volume in each machine to increase the rotation ratio of lot processing.
This method involves setting up two kinds of limitation values of work volume for each step, one being a limitation value for stopping receipt of lots in the step concerned, and the other a limitation value for restarting the stopped step. When the work volume of the step exceeds the set value, receipt of lots in the step is stopped. When the work volume decreases to below the set up limitation value, receipt of lots is started again. Thus, the work volume is controlled. However, in this case, since it is impossible to grasp process schedules for respective steps in each lot, delivery date management and TAT (turn-around-time, i.e., number of days till completion of the lot) control are difficult.
On the other hand, in a production line called ASIC-LSI (Application Specific Integrated Circuits-LSI), as shown in FIG. 4, there exist many kinds of lots whose priority of process and importance differ from lot to lot. For example, special order products and products of special specifications are small in throughput, and require high designing and production costs, which assigns high importance whereas mass production products such as memories, and general-purpose products are assigned low importance. Among ASIC-LSI, LSIs using full custom cells, or standard cells, are given high importance since specialized mask patterns must be used from the substrate step. In the case of Gate array LSIs, however, common substrates are used, and thus are given low importance originally but high priority is assigned after they enter the wiring step using a specialized mask pattern. On the other hand, the priority of a lot is often changed as a result of monitoring of the allowance in time till the delivery date, and thus varies according as the progress of the processing of the lots. Therefore, the priority of lots changes along with the progress of the processing of the lots.
The time passing from the input of products to their completion, i.e., turn-around-time (TAT), varies depending on the number of lots input in the line. If products of different priorities coexist, those having lower priorities are preceded by those having higher priorities, and process of the lower priority products is deferred. For this reason, mere judgment as to whether or not it will be in time for a delivery date by estimating progress for each lot based on the number of lots worked-in in the machine and process time, as has been conventionally adopted, results in failure of precise estimation of date on which products are completed. Then, in order to control the date of completion of lots having higher priority or importance (hereinafter, referred to as "TAT-controlled lots") so as to be in time for a delivery date, and in addition, to prepare a key information for collecting data, it is necessary to monitor progress of lots, and executing not only a regular or periodic scheduling, e.g., once a day, but also an irregular scheduling to make up difference or gap in progress due to troubles of the machine or delay in progress in order to gradually renew the process schedule. That is, the progress is monitored, the priority as to which lot should be processed first is changed based on the process results thus obtained, and the remaining scheduling other than has already been processed must be executed taking into consideration various factors such as working period of machines (information on disorder, etc.), maintenance conditions, and working conditions of operators. To perform a production management using such a scheduling, all the lots in the line must be divided into steps, judgment must be made as to whether machines (in some instances operators in charge) are available, and a process schedule reflecting priority of each lot must be executed. In particular, in the production of LSIs, many of the steps require continuity with preceding steps (i.e., must be processed within a predetermined time), and hence in order to accurately estimate a delivery date, it is necessary to precisely execute a process start time schedule for each step in each lot with checking which time range is actually available for operation.
However, when it is intended to execute such a process schedule, there exist many combinations of machines and operators and combinations taking continuity with subsequent steps into consideration since there are a plurality of machines of the same kind. Therefore, in a production management system, memory size and calculation time for executing a schedule allocating each step in all the lots while checking working conditions of machines and operators (non-working period range) increases with increased numbers of lots and machines (further including number of operators, if operators are to be taken into consideration), and in particular, use of a large number of lots requires a huge volume of memory and a long calculation time.